Method of hardening work in an electric glow discharge



June 22, 1965 a. BERGHAUS Erm. 3,190,772

METHOD OF HARDENING WORK IN AN ELECTRIC GLOW DISCHARGE Filed Feb. 10, 1961 2 Sheets-Sheet l a If l lfl Z INVENTORS rd Baas June 22, 1965 B. BERGHAUS EI'AL 3,190,772

METHOD OF HARDENING WORK IN AN ELECTRIC GLOW DISCHARGE Filed Feb. 10, 1961 2 Sheets-Sheet 2 FF. B zzkad Br (ms ATTORNEYS United States Patent METHOD OF HARDENING WORK IN AN ELECTRIC GLOW DISCHARGE Bernhard Berghaus, Grand Hotel Dolder, and Hans Bucek, Toblerstrasse 54, both of Zurich, Switzerland Filed Feb. 10, 1961, Ser. No. 88,497 12 Claims. (Cl. 148-16) Hardening processes may be performed in an electric glow discharge of which the characteristic is that diffusion processes are accelerated in the ionized gas atmosphere, substances penetrating into the surface of the work from the gas owing to the ion bombardment. The articles to be treated are commonly connected so as to operate as electrodes and will therefore be covered by a glow while being heated to a predetermined temperature when the energy conversion is properly adjusted. A wide variety of hardening treatments may be performed since it is possible to select the composition of the gas atmosphere accordingly, by way of example, a nitriding hardening treatment in gases containing nitrogen, a boron or silicon treatment in gas atmospheres containing boron or silicon, and the like.

The basic conditions for the performance of surface treatments in a glow discharge are known. The energy conversion at the surface of the work to be treated is commonly selected so that the temperature is obtained during the glow discharge which is required for the specific process. Since the treatment is regularly performed in a cooled vacuum container, the parameters are, besides the electric voltage and the current required, the gas pressure, the type 'of gas and the environment of the object to be treated, the latter owing to the transmission of heat influenced thereby, mainly by radiation, to the surfaces of the surrounding space. No difficulty is presented in subjecting e.g. large objects, such as machine shafts, casting moulds and the like, to hardening by nitriding, by way of example in a nitrogen atmosphere, or in hardening the inner surface of long tubes. Nor are there any material limitations, apart from certain extreme cases and from the fact that the object must be suspended from some point or be conductively connected with some power supply in some manner so as to operate as an electrode, in respect of the shape of the work. The treatment of Work units of which all surfaces must be treated and which may not possess an untreated point of suspension owing to their application, requires certain steps which will here be described. At the same time, the new method also relates to the improvement of the treatment of parts of complex shape, the term complex here being employed to denote mainly a shape which comprises, besides solid geometrical configurations, very thin-walled extensions, ribs, points, hooks and the like or which is at the same time provided with bores of very different diameters, particularly where the absolute values of the dimensions are small.

The difliculties in the treatment of such parts in a glow discharge arise from the fact that energy must be supplied per unit area in order to obtain the predetermined temperature for the performance of the diffusion process,

which may, under certain circumstances, be too large and cause, in conjunction with the occurrence of the known hollow-cathode effect, certain portions to be overheated. It is therefore'necessary on the one hand to reduce the glow load :of such work and, on the other, to maintain the diffusion temperature required. It has therefore been proposed to employ external heating, i.e. to heat the work to a certain temperature egg. by resistance heating, and then to apply only a relatively low-energy glow discharge to produce an ionized atmosphere and to start and accelerate the diffusion process. This method is basically practicable but it entails great practical disadvantages since the highly aggressive glow discharge which attacks any 2 insulating body, vaporize metals etc., can hardly be reconciled with the design requirements of an extensive installation of heating elements, insulations, supports and the like. Where voltages are somewhat higher, glow discharges will appear at theheating elements themselves since they, or at least their electric connections, are again located in the vacuum. In practice such arrangements have not been satisfactory. The position is entirely different when the method according to the present invention is applied, which resides in the fact that heat generation is effected-as usual with glow processesby a correr spondingly effective gas discharge, while the energy supplied for the work to be treated is divided into two portions of which the larger is employed for heating while the smaller portion is applied direct to the work surface to produce an ion bombardment. It should here be pointed out that fundamental differences exist in respect of the known arrangements with external heating by means other than glow discharges. While heating by means of a glow discharge has for its primary object the generation of heat, it also results in an ionization of the entire gas atmosphere employed which is so pronounced that entirely different conditions will obtain than with an arrangement which employs e.g. resistance heating. The performance of the envisaged hardening process is favourably affected by the presence of stimulated conditions in the gases since an additional rigid separation of the heating operations from the present ionization processes of a predeterminedly lower energy is not performed in the method here discussed. Apart from the electrical conditions, certain factors must be considered which relate to the uniform heating of all units. Accordingly, certain electrode arrangements are obtained which must answer all requirements, on the one hand in respect of heat generation and reflection, and, on the other hand, asregards the predetermined distribution of the glow loads. Related thereto are circuiting measures which are of importance mainly with the practically considered polyphase supplies.

The method of hardening work units of complex shape, or work units requiring treatment on all sides, which are connected so as to operate as electrodes in an electric glow discharge and heated by means of an electric gas discharge in an ionized gas atmosphere in a cooled vacuum container, is characterized by the fact that the work is arranged under heat radiation within a portion of the vacuum container heated by the glow discharge, the said radiation being practically identical for all units, and then subjected to a glow load which is smaller than the energy required for heating to the temperature of treatment. The total energy required for generating and maintaining the temperature of treatment is applied by the glow discharge while only part of it is operative directly on the work, preferably approximately 1050% while 5090% is indirectly employed for heating the work in that the heat liberated at auxiliary electrodes is transmitted to the work by conduct-ion and/or radiation. In order to heat the objects to be treated uniformly, a portion of the vacuum container employed is advantageously separated from the rest of the container by a metallic wall, this wall being connected so as to operate as the electrode and heated by a glow discharge. Appropriate arrangement of the work to be treated on the inside of this electrode will readily achieve suflicient uniformity of temperature distribution over all work units, a corresponding portion of the interior being kept free to accommodate a further auxiliary electrode in the vicinity of the objects. In such spatial distribution, it is readily possible to supply, from the outside, a certain relatively large glow energy to the first auxiliary electrode and the inside electrode is supplied with a smaller energy relative thereto. This energy, which is obtained through a glow discharge and which therefore contributes to the heating of the objects, may be varied within wide limits and it is generally bound to only the lower limit which is determined by the requirement that all surfaces to be treated must be covered by a glow. In this connection, it should be mentioned that, with practically applicable processes and in the gas pressure range applicable, hardening treatment cannot be performed unless the surface is at the same time covered by a glow. By way of example, the well-known fact is here mentioned that nitriding of a steel object will not occur in vacuum even when the temperature and the nitrogen supply are adequate.

It is therefore a further characteristic of the method that the work is arranged within an electrode forming a hollow space, which is heated by a glow from the outside while another electrode produces, in the interior, a glow of such intensity that all surfaces to be treated are at least covered.

The electrode defining the space portion accommodating the objects to be heated in its interior and being heated outside by glow energy supplied to it may fundamentally be of any configuration. In practice, considerations arise which are determined mainly by the possibilities of arranging the work to obtain uniform glow coverage and indicate the shape of a cylinder symmetrical in respect to an axis. The distribution of both heat and glow can readily be checked and controlled. The cylindrical shape not only offers the advantages of a readily visible arrangement, it is also advantageous in respect of heat radiation to the outside towards the cooled walls of the vacuum container. If desired, the application of radiation shields for the purpose of further reducing radiation losses may be readily solved thanks to the simple geometric conditions. The uniformity of heating of the work arranged inside is ensured in a hollow space having evenly heated walls, and one need only make sure that the objects are not arranged in the immediate vicinity of the openings of the larger exterior electrode required to position the interior auxiliary electrode. According to application, shape of the work, number of articles etc. the cylindrical electrode may also be closed at one end. A further characteristic feature is therefore found in the fact that the electrode is cylindrical and has at least one side thereof designed so as to have its end surface open while the work units to be treated are arranged at the inside wall of the cylinder, leaving free a marginal area in the vicinity of an open end surface.

In such an arrangement it will then be possible separately to adjust the heating on the one hand, and the generation of the ionized gas atmosphere required for the hardening process on the other, each at a different predetermined power level. The heating power is supplied to the preferably cylindrical electrode from the outside and the additional glow energy for the generation of the ion bombardment of the work units is supplied in their interior. This setting of the energies to be adjusted relatively is performed by appropriate selection of the electric voltages applied between the electrode carrying the work and other exterior electrodes, preferably the container wall, and between the work and the interior auxiliary electrode. It is therefore characteristic that a larger voltage is applied between the vacuum container and the cylindrical electrode for the supply of the glow electrodes, and a smaller voltage between the work and a further auxiliary electrode adjacent to the work.

The equipment for glow treatment may be supplied with either direct current or alternating current. Both types of operation are known. It has been proposed to employ both direct current and alternating current in a single recipient simultaneously or consecutively. From the practical point of view of securing economical operation, however, this procedure should not be recommended. Direct-current operation as such is quite useful since rectifiers of satisfactory efliciency can be designed. But in practice, direct operation with polyphase alternating current has proved to be effective on the one hand because the supply equipment is simpler and, on the other, because the accommodation of a greater number of articles per batch is simplified. In addition, the entire electrode arrangement in the vacuum containers can be made simpler and more reliable in operation since additional auxiliary electrodes may be dispensed with while the vacuum container itself need not be used as an electrode. A voltage supply system symmetrical to ground may be designed which is desirable in respect of both the behavior of the glow discharge in the recipient and safety considerations. The method is therefore further characterized by the fact that, when the glow electrodes are supplied with polyphase currents, a larger voltage is applied between the number of cylindrical electrodes, which corresponds to the number of phases, containing the work, and a lesser voltage between the work units and the adjacent auxiliary electrodes.

The technology of treating work in electric glow discharges teaches that measures must be taken in order to secure a corresponding change in the treating gases. It is not sufiicient merely to maintain a constant gas glow in the vacuum container; the convection, hardly present in negative pressure, at the parts to be treated must be adequately supported in order to avoid a reduction of reactive substances in the atmosphere of treatment. In this respect, a periodical change of the electrical operating data of the glow discharge has proven to be exceptionally advantageous, and owing to the local variation of the gas temperature and ionization, a sufficient change in the gas is obtained. In the present arrangement of the work within a heating electrode, this step assumes particular importance, and the periodical variation of the glow discharge inside on the work is most easily obtained, by way of example, by a rhythmically recurrent brief stoppage of the discharge. Since the energy supplied to the electrodes adjacent to the work units is preferably adjustable, the method according to this invention is further characterized by the fact that, in polyphase circuit arrangements, the supply of the auxiliary electrodes adjacent to the work units is taken from an adjustable tap on the windings of a transformer and interrupted in cyclical permutation to obtain periodic intensity variations of ionization.

Depending on the shape or size of the work it may be desirable not to provide several heating electrodes for the treatment and to employ only one large electrode instead. Even so a polyphase A.C. operation is advantageous without going to the length of using a rectifier and supplying direct current. Such solutions are possible and have proved themselves in practice. It is therefore a further characteristic feature of the invention that, with threephase arrangements, three exterior electrodes supplying the heating energy are provided opposite a single cylindrical electrode holding the work units in its interior, the said electrodes being connected to the phases and the cylindrical electrode being periodically connected, in cyclical permutation, to the individual phases.

The particular advantage of the method according to this invention resides in the fact that bodies of complicated shape, which are correspondingly subject to overheating, or bodies with surfaces easily damaged, may be treated with perfect results. In order to illustrate the difficulties arising in certain critical cases, mention is made of an example as encountered e.g. in the hardening treatment of small sewing machines, ofiice equipment etc. components. If such bodies cannot be suspended,, e.g. in wire loops, but are subjected to the action of the glow discharge while resting on a metallic support, it is easily possible for a cylindrical or spherical portion of an object to rest on a flat surface. Viewed geometrically, this will result in that all distances between zero and a magnitude depending on the absolute values of the dimensions are encountered from the point of contact between the two metal surfaces of the support and of the cylinder or sphere. It being known that the hollow-cathode effect between two electrodes of the same polarity in a glow discharge may result in a substantial concentration of energy, excessive heating may occur at a certain gas pressure, or possibly only a locally limited injury to the surface. The extent of these occurrences is determined by the amount of energy employed in the glow discharge.

Since it is possible to reduce the glow load on the work to a minimum, the undesirable damage to the surface may also be avodied, the hardening process being maintained in the still ionized atmosphere owing to the temperature generated elsewhere and transferred to the work. While this condition must be met with sensitive work, a maximum degree of efliciency in the transformation of electrical energy supplied should be sought in order togenerate the temperature of the work. Means should therefore be employed which enable at least a major portion of the glow effect to be converted at the points of the electrodes suitable for heating. Since the electrodes, and particularly the preferably cylindrical heating electrode which encloses the work units, may readily be of robust design and since possible surface effects need not be heeded as is the case with the objects to be treated, this electrode may be placed under a very high specific load. In this case, the hollow-cathode effect is advantageously utilized by adequately designing the outer surface of the heating electrode. The relationships between gas pressure and electrode geometry being known for a given type of gas, the method is further characterized by the fact that the electrodes to which glow energy is supplied to heat the work have their surfaces provided with spaced hollows, such as bores, slots or grooves, or which the dimensions are adjusted, in view of the gas pressure applied in the treating chamber, for the formation of hollowcathode effects.

In connection with the cited example of small cylindri cal and spherical bodies, a further practical problem arises, i.e. the treatment of such parts on all sides. The work units are commonly suspended from wire loops or slipped over corresponding supports where bores are present. These methods cannot be applied to work of which all surface areas must be subjected to a hardening treatment. This condition can be met only if the parts are kept in motion in the glow discharge so that all points, including those used to rest on the electrodes, are consecutively subjected to the glow light. In such arrangements the work units assume continuously changing positions relative to the other units and to the supporting electrode. Consequently it is impossible to avoid areas with relative distances between electrodes of similar polarity which cause substantial concentrations of energy in the slit-type spaces at the gas pressure present. Even if they are of a temporary nature only and will vanish when the work changes its position, they may cause a reduction of the surface quality of sensitive parts. This problem, too, may be solvedby the present method in which too high a glow load is not reached. Perfect results may be obtained especially where the moving Work units are treated on all sides. It is therefore a further characteristic feature that the cylindrical heating electrodes are arranged in the vacuum container so as to be rotatable or periodically swivelable, which causes the work units to assume changing positions relative to the wall.

A number of embodiments of the invention are repre- K sented in the drawings in which:

FIG. 1 is a diagrammatic view of a basic arrangement of the electrodes and work units;

FIG. 2 shows an example of a simple design of an installation with a polyphase supply;

FIG. 3 shows an example of the circuit arrangement necessary for polyphase operation using three treatment electrodes with work units;

FIG. 4 is a circuit example for an arrangement for polyphase operation which employs only one heating electrode, and

FIG. 5 shows two examples of the design of the wall of the electrode which encloses the work units in order to obtain the maximum energy conversion.

. FIG. 1 diagrammatically shows a vacuum container 1, the usual means of producing the vacuum by suction at 2 and the supply of controlled quantities of fresh gas at 3 not being shown in greater detail. The heating electrode which carries and encloses the work units is shown as a cylinder cut open. In this embodiment, the cylinder has its top end open While its lower end is closed by the base plate 5, leaving a central opening. For the sake of clarity, a sector of the cylindrical hollow space is cut away so that the space portion 6 in which e.g. a plurality of small work units may be accommodated on suitable supports becomes more clearly visible. Arranged adjacent to this space portion 6, in the present case centrally in the longitudinal axis of the cylinder, is an auxiliary electrode 7. Provided in the lid of thevacuum container 1 are two insulating current lead-ins 8 and 9 which at the same time perform the function of a supporting structure for all interior elements in the container and which are protected against the destructive influences of the glow discharge by means of the gap arrangements known per se. The circuit outside the vacuum recipient is complemented by the two voltage sources diagrammatically indicated at It and 11. In the present case these are two A.C. sources, but D.C. may also be supplied in which case only certain conditions in respect of polarity must be complied with. In the operation of this arrangement, the following conditions obtain: the voltage source 10 is connected between the heating electrode 4 and the wall of the vacuum container 1. The glow formed at the electrode 4 will preferably spread, owing to the electrode geometry, over the outside of the cylindrical hollow body and, depending on the gas pressure applied, at best cover the outside of the end surface 5 and a small marginal portion on the inside of the top opening of the electrode 4. The energy supplied, which'can be adjusted by the electrical voltage applied, is selected in such a manner that the hollow electrode 4 will assume the predetermined temperature for the hardening process to be performed. As the work units are arranged within the hollow space of the electrode 4, they will assume the same temperature as the electrode wall since the metal-to-metal contact between the points of contact will transfer heat on the one hand while the heat radiation inside the hollow space is largely uniform and provides for adjustment on the other. In order to ensure a practical degree of uniformity, care must be taken that the heating zone 6 suited to the work units will leave free the marginal portions of the hollow space which are characterized by an excessive heat radiation -to the outside. The example according to FIG. 1 therefore shows that a larger marginal portion is kept free towards the free end surface on top while the heating zone 6 is closely approached towards the closed end surface 5 and, respectively, that the space for the accommodation of work pieces may be directly utilized as far as this end surface. This example has been selected to illustrate conditions in open or closed, or partly closed arrangements, and it should in this connection be mentioned that FIG. '1 does not show the correct relationship between the length and the diameter of the cylindrical hollow space, for the sake of clarity. In practice, a longer cylinder would be employed. Again, a bottom completely closed and a closure wall having an opening similar to the end surface 5 shown, would provide particularly uniform heating and, respectively, radia tion conditions for a heating zone which could then be more extended. The temperature distribution for work units will, in accordance With the factors now discussed, be readily controlled in all cases of practical application, and the absolute value of temperature is adjusted by means of the glow discharge. In order that the Work units may be subjected to a hardening treatment requires, as has been stated above, the maintenance, apart from the temperature, of an ion bombardment, i.e. the work units themselves must be covered by a glow on the surfaces to be treated. To this end the second auxiliary electrode 7 is provided which is adjacent to the work units and which passes a voltage to the work piece which is supplied by the generator 11. This voltage should be high enough for the work units to be covered by a glow at least on the surfaces to be hardened. With the geometrical arrangements disclosed and in application of two different voltages, it is readily possible to maintain the practical operating conditions required. Naturally two separate A.C. sources would not be employed in practice with the embodiment selected, but e.g. two adjustable taps on the secondary winding of a transformer, the center tap point being connected with the heating electrode. If D.C. is supplied, the hollow heating electrode with the work units accommodated in its interior would form the cathode in respect of the wall of the recipient and of the auxiliary electrode 7, which would therefore constitute an auxiliary anode.

To clarify conditions, some figures are cited for a nitriding hardening process to be performed, by way of example, in a gas atmosphere containing nitrogen. The steel bodies to be ni-trided are located in the interior of the electrode which forms the heating chamber, and arranged in such a manner that a portion of the space around the auxiliary electrode located adjacent to the work units remains free. Experience in the art shows that the spacing is not by any means critical, which is particularly advantageous if the said continuous change in the position of the work units due to the movements of the heating electrodes is applied for the treatment of all surfaces. After filling the container with a gas, nitrogen by way of example, a 50/50 nitrogen-hydrogen mixture, or cracked ammonia, or ammonia, a gas pressure of 2-50 Hg, advantageously of 3-5 Hg is produced. When a voltage is applied between the heating electrode and the auxiliary electrode located inside, a glow will appear on the work units and in their vicinity, and if its intensity is such as safely to cover the work, a temperature of ap prox. 180-280 C. would be produced in the interior of the heating electrode without additional energy supply.

These data refer to the usual conditions in respect of packing the work and cooling, which is again dependent on the radiating surface of the outside of the heating electrode. For the performance of a nitriding process, the work units must be heated to a temperature between 500 and 600 C., and the principal portion of the heating energy must be additionally supplied to the heating electrode. For this purpose, the glow is generated on the outside and the energy transfer between this electrode and the container wall here forming the auxiliary electrode is raised so that nitriding temperature is obtained. The energy supplied may be determined in accordance with simple principles since it is known that the radiated heat with the steel materials exclusively used is in the magnitude of 1.5 watts per cmF. In order to maintain the temperature permanently, the same specific energy must be supplied as well. This constitutes the entirety of the determining factors, but it may be stated that the voltages to be applied are in the range between 400 and 550 volts, the energy converted commonly amounting to many kw. depending on the size of the heating electrodes. The energy supply according to the present principles applies to the usual design of the vacuum container with cooled walls. However, the design of the recipient may also provide for partial cooling only, by way of example where the seals against the outside atmosphere are located. The application of reflecting radiation screens is possible as well. In such cases the above magnitude of 1.5 watts does not apply to the specific energy, but a value reduced in accordance with the prevailing conditions.

Under practical operating conditions, the process is performed not with single-phase AC. or DC, but with polyphase A.C., preferably three-phase current. FIG. 2 is an example of the simplest possible circuit. It shows only the electrode arrangement and the principle of power supply without indicating the vacuum container which is naturally present. According to the three-phase arrangement, three cylindrical heating electrodes 12, 13 and 14 are provided of which the essential design corresponds to the details shown in FIG. 1. These three electrodes are connected to the three phases of a starcircuit transformer 15, 16 and 17. The full interlinked voltage is applied between the three cylinders which, among themselves, operate as glow electrodes. It is therefore not necessary to include additional auxiliary electrodes or the container wall in the circuit. Under these operating conditions only the heat radiation impinging on the wall is transmitted to the outside and no electrical energy is converted as a loss on the wall of the recipient. In order to produce a low-energy glow discharge on the work units, the interior electrodes 18, 19 and 20 are provided which are jointly connected to the neutral point 21. This results in that the phase voltage is applied between the cylinder 12 and the electrode 18, and between 13 and 19 and, respectively, 14 and 20, the said voltage being smaller by the factor /3 than the interlinked voltage of the individual heating electrodes relatively to one another. It has been found that this procedure causes the minimum-but adequateglow effect to be obtained on the work units inside the heating electrodes. The further advantages of this arrangement reside in its symmetry to the vacuum container and, accordingly, to ground. The entire gas atmosphere in the container possessing considerable conductivity owing to ionization, it is hardly possible, in the event of pronounced asymmetry, to avoid currents which flow from the electrodes to the container wall, which is metallic throughout, and thence back to the other electrodes. The example here shown therefore constitutes a simple arrangement, dependable in operation, of a polyphase supply.

FIG. 3 shows a circuit diagram for a three-phase power supply employing three cylindrical heating electrodes as in FIG. 1 and provided with some facilities required in practical operation. Three electrodes 22, 23 and 24 are shown in cross-section, the space portions in their interior, which are suitable for accommodating the work units, being marked by hatching. In contradistinction to the previous example, the auxiliary electrodes 26, 27 and 28 arranged in the interior are not jointly connected to the neutral point of the three-phase system but, via three switches, to the taps 29, 30 and 31 of the secondary windings 32, 33 and 34 of the three-phase transformer. The heating electrodes are connected to further taps 35, 36 and 37 of the same transformer winding. This circuit arrangement shows that the voltage between the objects and the auxiliary electrodes is not limited to one value but may be varied within wide limits. With this arrangement, it is presupposed as usual that the associated taps are jointly and simultaneously displaced on all three transformer legs. It should further be pointed out that not only the voltage range may be covered at the ratio between zero and phase voltage as might be assumed from the diagram, but that still larger voltage differences may be obtained between the auxiliary electrode and the heating electrode if the taps 29, 30 and 31 are not connected to the same phase as the associated cylinders, but applied to a cyclically permuted other phase. The interior electrodes are connected to their respective connections via contactors 38, 39 and 40. The latter are actuated by a suitable switching member, e.g. an electronic impulse generator 41, actuation being performed periodically in a uniform rhythm. Dif- 9 ferent possibilities are available therefor; by way of example, the arrangement has proved satisfactory in which two of the contactors are closed and one opened, this condition changing cyclically in order to create, on an average during extended periods, identical conditions for all three phases. This arrangement causes, by way of example, a glow to be present on the objects to be treated for two seconds at one-second intervals. This rhythm is continuously repeated. The periodical changes in ionization cause a parallel volume alteration in the gas portions involved, this effect being at the same time supported by thermal effects. Partial volume and pressure changes, however, result in corresponding displacements of gas volumes, appropriate mixing and, respectively, the necessary supply of reactive substances to the Work units treated. This periodical change in the operating data of the glow discharge has proved satisfactory in practice and greatly contributes to the uniformity of the results in the hardening processes to be performed. Further possibilities of variations are not described in greater detail; the supply of the heating electrodes may, by way of example, be incorporated in the pulsing operation.

FIG. 4 shows a circuit diagram of the possibilities presented if only one heating electrode is to be used with polyphase supply, by way of example if a plurality of work units or units of large dimensions are to be handled. As in the other figures, the three-phase supply is here indicated only by the secondary winding of a transformer, the primary winding being omitted. The three windings 42, 43 and 44 are connected to three auxiliary electrodes 45, 46 and 47 which face a cylindrical heating electrode 48 at a suitable distance and which are shifted by 120. As in the previous examples, the work units are accommodated in the interior of the electrode. Arranged opposite to them is the inner auxiliary electrode 49 which is connected, as in FIG. 2, to the neutral point 50. Naturally, this arrangement may be extended by employing other circuit designs. The heating electrode 48 may be connected to the phases via three contactors 51, 52, 53 while an electronic impulse generator 54 ensures that only one cont actor is closed at a time. The three contactors 51, 52 and 53 are again controlled by the unit 54 in cyclical permutation and actuated at short intervals. This arrangement provides that two of the auxiliary electrodes 45, 46 and 47 conduct the full interlinked voltage to the heating electrode at any given time, the cyclical permutation ensuring an entirely uniform heating of and, respectively, glow effect on the said electrode.

As stated previously, it is desirable preferably to pass the heating energy supplied to that electrode which is used to heat the work units in its inner hollow space. The utilization of the hollow-cathode effect proves to be favourable in respect of both the radiation conditions and the operatively desirable use of low electrical voltages. FIG. shows two embodiments of the design of the wall of the heating electrodes. Utilization of this elfect is obtained merely by providing bores 56 in the wall 55 which are regularly spaced in order to provide for uniform heating of the electrode. This method may be applied if the wall is formed of sheet metal which is not too thin since a useful degree of the hollow-cathode effect depends on a length of the bore which is not too small. To give figures as an example, an effect can be obtained within the ranges of the gas pressure previously given provided that the thickness of the sheet metal is above 3 mm. and the bore has about the same diameter or does not exceed 4 mm. where heavy sheet metal is employed. Small bores should be used where it is e.g. preferred to provide the cylindrical electrode with grooves applied from the outside, this method of manufacture being simpler, particularly where the wall thickness is greater. There are still simpler methods of designing the wall. By way of example, many types of perforated sheet metal are available commercially which are provided with regularly spaced openings over their surface, the wall portions of the said openings being parallel or substantially parallel and possessing suitable dimensions. The right-hand portion of FIG. 5 shows a cross-section of an embodiment as manufactured and sold commercially under the name expanded metal or the like. The sheet metal portions 57 seen in cross-section are parallel in anarea which may be utilized for the present purpose/ The hollow spaces are present in regular distribution throughout the surface. The example is naturally not limited to this embodiment; any structure may be employed which is provided with hollows of adequate dimensions.

We claim:

1. A method of treating metallic work pieces at an elevated temperature by means of an electric glow discharge, comprising the steps of: placing said work pieces in an ionized gas at subatmospheric pressure; substantially surrounding said work pieces with at least one electrode; creating a first high-energy electric glow discharge on said electrode to heat the same whereby only a part of the heat energy necessary for creating said elevated temperature is radiated to said work pieces; and creating a lowenergy'electric glow discharge on those surfaces of said work pieces which are to be treated to produce ion bombardment, penetration thereof and the remainder of the heat energy necessary for creating said elevated temperature.

2. The method defined in claim 1 wherein said electrode is a hollow body, said high-energy glow discharge being on the outer surface thereof, said low-energy glow discharge being between the inner surface of said body and said work pieces.

a 3. In the method defined in claim 2 wherein said body is a hollow cylinder having at least one open end, the further step of holding said work pieces in said cylinder at a substantial distance from said open end.

4. The method of claim 2 including the further steps of confining said gas, electrode and work pieces in a conductive container; said high energy discharge being produced by applying a first high voltage between said container and electrode; said low energy discharge being produced by applying a second and lower voltage between said electrode and said work pieces.

5. The method of claim 1 including the further steps of producing said high energy discharge by polyphase current; providing a plurality of said surrounding electrodes, equal in number to the phases in said polyphase current; placing some of said work pieces in each electrode; connecting each phase of said current to a respectively different electrode; and producing said low energy discharge between said work pieces and a further electrode associated with each surrounding electrode.

6. The method of claim 5 including periodically interrupting the voltages for said high energy discharge, cyclically in phase with said polyphase currents.

7. The method of claim 1 wherein said electrode is a single cylindrical body having said work pieces therein; said high energy discharge being produced between said cylindrical body and three separate electrodes external thereto; and applying three-phase voltage to said three electrodes, an individual phase being applied to each external electrode.

8. The method of claim 1 including the step of providing the surface of said surrounding electrode with a multiplicity of openings to provide local hollow-cathode effects at the particular pressure of said gas.

9. The method of claim 1 including the step of rotating said work pieces to expose different surfaces thereof to radiant heat and to said low energy discharge.

10. A method according to claim 1 in which said metallic work pieces are hardened in an ionized gas atmosphere by ion bombardment.

11. A method according to claim 10 in which the ionized gas atmosphere contains a member selected from the group consisting of nitrogen, boron and silicon.

2,266,735 12/41 Berghaus et a1.

7 12 Berghaus et a1. Nelson 315-111 Kilpatrick 315-111 Bucek 148-166 Berghaus et a] 148-16.6

DAVID L. RECK, Primary Examiner.

ARTHUR GAUSS, Examinen, 

1. A METHOD OF TREATING METALLIC WORK PIECES AT AN ELEVATED TEMPERATURE BY MEANS OF AN ELECTRIC GLOW DISCHARGE, COMPRISING THE STEPS OF: PLACING SAID WORK PIECES IN AN IONIZED GAS AT SUBATMOSPHERIC PRESSURE; SUBSTANTIALLY SURROUNDING SAID WORK PIECES WITH AT LEAST ONE ELECTRODE; CREATING A FIRST HIGH-ENERGY ELECTRIC GLOW DISCHARGE ON SAID ELECTRODE TO HEAT THE SAME WHEREBY ONLY A PART OF THE HEAT ENERGY NECESSARY FOR CREATING SAID ELEVATED TEMPERATURE IS RADIATED TO SAID WORK PIECES; AND CREATING A LOWENERGY ELECTRIC GLOW DISCHARGE ON THOSE SURFACES OF SAID WORK PIECES WHICH ARE TO BE TREATED TO PRODUCE ION BOMBARDMENT, PENETRATION THEREOF AND THE REMAINDER OF THE HEAT ENERGY NECESSARY FOR CREATING SAID ELEVATED TEMPERATURE. 